Block the messenger

Dear All,
We are using the ipcop firewall & squid proxy server in the network.. i block the yahoo.com & some site..
But i cannot block the messenger in this..
So can u help me how to block the messenger... (Yahoo, Msn & rediff)
Regards
Bhavik

What linux r u using, .. i do this on my network,
i am kind of a jack of all trades, so offhand i cannot give you the practical instructions, but will explain how i do it.
Assumtions :
1. all browsing is via Proxy,
2. the messengers are unaware of your proxy.
you will firstly need to know what ports are used by the messengers for connecting to the service.
for example yahoo users ports 5001 or 5050 something, but all ads in yahoo messenger use port 80. you can get this info from http://www.portforward.com/you could alternatively load a windows firewall like Sygate and check what ports are used at login etc. and get the info first hand.
Now once you have these details you can drop packets that are destined to those particular ports.
its that simple but do your homework before you implement.
I have succesfully blocked the inpage google chat in gmail via my proxy.
Hope this info helps.
Cheers